Light emitting diodes (LEDs) can be fabricated by epitaxially growing multiple layers of semiconductors on a growth substrate. Inorganic light-emitting diodes can be fabricated from GaN-based semiconductor materials containing gallium nitride (GaN), aluminum nitride (AIN), aluminum gallium nitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN). Other appropriate materials for LEDs include, for example, aluminum gallium indium phosphide (AlGaInP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), diamond or zinc oxide (ZnO).
Especially important LEDs for this invention are GaN-based LEDs that emit light in the ultraviolet, blue, cyan and green regions of the optical spectrum. The growth substrate for GaN-based LEDs is typically sapphire (Al2O3), silicon carbide (SiC), bulk gallium nitride or bulk aluminum nitride.
Typical epitaxial growth methods for thin semiconductor layers include chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). These growth methods are relatively slow and expensive.
In order to produce LED semiconductor layers with greater structural integrity and to reduce current spreading issues, there exists a need for LEDs with at least one thick semiconductor layer. Epitaxial grow methods exist that can grow thicker layers, including liquid phase epitaxy (LPE), vapor phase epitaxy (VPE) and hydride vapor phase epitaxy (HVPE).
Thick layers of GaN-based materials are, however, difficult to etch, especially anisotropically. Etch rates on the order of hundreds of nanometers per minute limit the feature thicknesses that can be economically rendered in these materials. As such, the use of mechanical means such as dicing and laser scribing are typically used even in thin devices. There exists a need for an improved high-speed method for etching and patterning LEDs fabricated with at least one thick semiconductor layer.
Thermal considerations are very important for LEDs, which generate a significant amount of heat during operation. The heat lowers the light output and operating lifetime of the LED. As LED sizes become larger, such heating effects become more important and can seriously degrade the light-output performance and lifetime of the LEDs.
Sapphire is a poor thermal conductor. If the GaN-based semiconductor structure is grown on sapphire, it is desirable to remove the semiconductor structure from the sapphire growth substrate and bond the semiconductor structure to a second transfer substrate that has high thermal conductivity. Cheung et al. in U.S. Pat. No. 6,071,795 disclose a method for separating a thin film of GaN from a sapphire substrate. The method includes irradiating the interface between the GaN film and the substrate with light that is transmitted by the sapphire and strongly absorbed by the GaN film. At the interface, the irradiation causes the decomposition of GaN into gallium metal and gaseous nitrogen. An example irradiation source is a pulsed krypton fluoride (KrF) excimer laser operating at 248 nanometers. The laser beam is raster scanned across the sample. The 248 nm light is transmitted by the sapphire substrate and strongly absorbed by GaN thin film layer. Following irradiation, the exposed sample is heated above the melting point of gallium (30 degrees Celsius) and the substrate and GaN layer are separated. Any gallium residue remaining on the GaN layer after separation can be removed using, for example, a 50:50 volumetric mixture of hydrogen chloride (HCl) and water (H2O). In order to handle the thin GaN layer and prevent the layer from breaking, the side of the GaN layer opposite the growth substrate is bonded to a second transfer substrate before the subsequent irradiation and separation steps.
Cheung et al. do not disclose using a raster-scanned pulsed laser beam to both separate the semiconductor layer from the substrate and to simultaneously form surface features that can be used as light extraction elements in LED devices fabricated from the semiconductor layer. Cheung et al. also do not disclose methods to etch thick semiconductor layers.
Kelly et al. in U.S. Pat. No. 6,559,075 disclose a method for separating two layers of material by exposing the interface between the two materials to electromagnetic radiation to induce decomposition of one of the materials at the interface. One of the layers can be a GaN-based semiconductor material and the other layer can be a substrate. An example electromagnetic radiation source is a pulsed, frequency-tripled, neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser operating at 355 nanometers. An example substrate is sapphire. The 355-nanometer light is transmitted by the sapphire substrate and absorbed by the GaN-based semiconductor material at the sapphire-GaN interface. The decomposition of GaN results in the formation of gallium metal and nitrogen gas.
Kelly et al. do not disclose using a raster-scanned pulsed laser beam to both separate the semiconductor layer from the substrate and to simultaneously form surface features that can be used as light extraction elements in LED devices fabricated from the semiconductor layer. Kelly et al. also do not disclose methods to etch thick semiconductor layers.
Park et al. in U.S. Patent Application Publication No. 20050227455 disclose a method for separating a layer of material such as GaN from a sapphire substrate. The method includes irradiating the interface between the GaN film and the substrate with laser light that is transmitted by the sapphire and strongly absorbed by the GaN film. An example laser is a pulsed krypton fluoride (KrF) excimer laser operating at 248 nanometers. In one optional method, the laser beam is formed into a line-shaped beam and the line-shaped beam is scanned across the sample. Park et al. do not disclose using a raster-scanned pulsed laser beam to both separate the semiconductor layer from the substrate and to simultaneously form surface features that can be used as light extraction elements in LED devices fabricated from the semiconductor layer. Park et al. also do not disclose methods to etch thick semiconductor layers.
It would be desirable to develop a method to form thick, substrate-free LED chips that did not need the original growth substrate or an attached bonding transfer substrate for structural support. Such substrate-free LED chips could be easily manipulated and bonded to other surfaces or leadframes in any desired pattern to form light emitting devices. As part of such a method, it would be desirable to develop processes to etch thick semiconductor layers. It would also be desirable to develop a method to both separate a semiconductor structure from a growth substrate and to simultaneously form surface features on the LED chips. The surface features can be used as light extraction elements for the LED. The combined separation and surface-feature-forming process would eliminate the separate step of forming light extraction elements after the removal of the LED chips from the growth substrate.